Performing a physical examination to assess characteristic features.

Choice A rationale:

Monitoring for gastrointestinal abnormalities is an important aspect of caring for a child with Down syndrome but is not a component of the initial assessment for diagnosing Down syndrome.

The diagnosis of Down syndrome is typically based on clinical features and genetic testing, not on the presence of gastrointestinal abnormalities.

Choice B rationale:

Performing a physical examination to assess characteristic features is indeed an essential component of the assessment process for diagnosing Down syndrome.

There are specific physical characteristics associated with Down syndrome, including a characteristic facial appearance, low muscle tone, joint laxity, and other features.

A physical examination helps confirm the clinical signs of Down syndrome, which can then be confirmed through genetic testing.

Choice C rationale:

Administering psychological tests to assess cognitive abilities is a valuable assessment for children with Down syndrome, but it is not a primary component of the diagnostic process.

The diagnosis is typically made based on clinical features and genetic testing.

Psychological tests may be conducted after the diagnosis to assess the child's cognitive abilities and inform their educational plan.

Choice D rationale:

Conducting DNA sequencing tests is not typically the first step in diagnosing Down syndrome.

The diagnosis is often based on clinical evaluation, including the physical examination of characteristic features, and confirmed through genetic testing such as karyotyping or chromosomal microarray analysis.